MIMO (Multi-Input Multi-Output) communications have conventionally been known as a technique capable of improving the frequency usage efficiency in a radio communication system. In the MIMO communications, multiple signal sequences are transmitted at the same frequency and time while each of the transmission side and reception side of radio signals uses multiple antennas.
In order to exert the performance of MIMO communications, transmission control needs to be adaptively performed in accordance with the state of a radio channel. For this reason, closed-loop control is introduced into a radio communication system into which MIMO communications are introduced. To put it more specifically, the reception side of radio signals generates channel information in accordance with characteristics of a radio channel (hereinafter, referred to as “channel characteristics”) and feeds back the channel information to the transmission side. Feedback of the channel information is required in a radio communication system employing an FDD scheme, in particular, in which there is no correlation in the channel characteristics between the uplink and downlink.
In recent years, coordinated base station MIMO communications (also termed as “coordinated multi-cell transmission/reception”) have attracted attention as a technique developed from MIMO communications. In the coordinated base station MIMO communications, multiple radio base stations communicate with a single radio terminal at the same frequency and time (refer to Patent Document 1, for example). Using antennas of multiple radio base stations, the coordinated base station MIMO communications can increase the number of antennas usable in MIMO communications, and thereby achieve an increase in the transmission speed and also an improvement in the reception quality as compared with conventional MIMO communications.